Construction machines, such as hydraulic excavators and hydraulic cranes, perform various work by means of a hydraulic drive system. For example, Patent Literature 1 discloses a hydraulic drive system 100 as shown in FIG. 5.
In the hydraulic drive system 100, a plurality of control valves 130 are disposed on a circulation line 120 extending from a pump 110 to a tank. Each control valve 130 is connected to a pilot operation valve 140 (operation device) including an operating lever, and controls the supply and discharge of hydraulic oil to and from an actuator 150 in accordance with an operating amount of the operating lever. The hydraulic drive system 100 adopts a configuration in which, when any of the pilot operation valves 140 is operated, the hydraulic oil discharged from the pump 110 is released to the tank without passing through the control valves 130.
Specifically, the hydraulic drive system 100 includes a bleed-off line 160, which branches off from the circulation line 120 at a position upstream of the control valves 130 and which extends to the tank. A bleed-off valve 170 including a pilot port is disposed on the bleed-off line 160, and a secondary pressure from a solenoid proportional valve 180 is led to the pilot port of the bleed-off valve 170.
The bleed-off valve 170 includes an unloading passage forming a part of the bleed-off line 160. As shown in FIG. 6, the bleed-off valve 170 is configured such that the unloading passage is opened when the operating amount of any of the operating levers exceeds a first setting value θ1, and the opening area of the unloading passage gradually decreases as the operating amount of the operating lever increases from the first setting value θ1.
On the other hand, each control valve 130 includes a center bypass passage forming a part of the circulation line 120. Each control valve 130 is configured such that the center bypass passage is open until the operating amount of the corresponding operating lever exceeds a second setting value θ2, which is slightly greater than the first setting value θ1, and the center bypass passage is rapidly closed when the operating amount of the operating lever exceeds the second setting value θ2. By blocking the circulation line 120 by the control valve 3 in this manner, the hydraulic oil discharged from the pump 110 can be released to the tank through the bleed-off line 160.